1. Field of the Invention
The disclosures herein relate to an electrochromic display device, a method for manufacturing the electrochromic display device, and a method for driving the electrochromic display device, and more specifically related to an electrochromic display device capable of independently displaying multicolor, a method for manufacturing such an electrochromic display device, and a method for driving such an electrochromic display device.
2. Description of the Related Art
Electronic paper has been increasingly developed as electronic media replacing ordinary paper. The feature of the electronic paper may be that its display device is handled as ordinary paper, which differs largely from related art display devices such as cathode ray tubes or liquid crystal display devices. For example, the electronic paper may be a highly reflective display device capable of exhibiting high white state reflectance and high contrast ratio, high definition, stable memory effects, low-voltage driven, thin and lightweight, and inexpensive characteristics.
Various types of display devices have been proposed suitable for the electronic paper. Examples include a reflective liquid crystal display device, an electrophoretic display device, and a toner electrophoretic display device. These types of display devices may be provided with layers of different color filters so as to display device multiple color. However, the color filters themselves may absorb light to lower the reflectance of the display device. Thus, the above types of display devices may have difficulty in producing multicolor images while maintaining certain white state reflectance and contrast ratio.
Meanwhile, as an example of a reflective display device without having color filters, an electrochromic display device may be given. The electrochromic display device utilizes electrochromism. The electrochromism is a phenomenon in display devices of materials that exhibit reversible color changes induced by the application of voltages. When a voltage is applied to the electrochromic display device, electrochromic materials undego reversible redox reactions to reversibly change color. The electrochromic display device is a reflective display device, has memory effects, and capable of being driven by a low voltage. Accordingly, the electrochromic display device has been extensively studied as one of the prospective candidates for the electronic paper display devices, from material development to device design.
Note that the technology of the electrochromic display device is based on the principles of redox reactions to develop or dissipate colors of materials. Accordingly, electrochromic responsiveness may be relatively low. For example, Japanese Patent Application Publication No. 2001-510590 (Patent Document 1) discloses an example of an electrochromic system utilizing the above principles of redox reactions. In this electrochromic system, electrochromic compounds are fixed near electrodes such that the system has a very rapid electrochromic effect. In the system disclosed in Patent Document 1, electrochromic response time was significantly improved from about ten seconds typically obtained in the related art electrochromic display devices to one second.
Further, since the electrochromism is an electrochemical phenomenon, electrochromic responsiveness or memory effects of color in the electrochromic display device may be largely affected by the performance of the electrolyte layer (e.g., ionic conductance) forming the electrochromic display device. The electrochromic display device including the liquid electrolyte layer formed by dissolving electrolytes into solvents may exhibit excellent electrochromic responsiveness; however, the electrochromic display device may exhibit inferior element strength or reliability.
However, a solid-state electrolyte layer or a gel-like electrolyte layer has been proposed to overcome such inferiorities. Specifically, the solid-state electrolyte layer formed of polymeric solid-state electrolyte has been proposed. The electrical conductivity of the polymeric solid-state electrolyte may be approximately 3 digits lower than an ordinary nonaqueous electrolytic solution. To overcome such low conductivity of the polymeric solid-state electrolyte, Japanese Patent Application Publication No. 63-94501 (Patent Document 2), for example, proposes a semisolid electrolyte layer obtained by dissolving polymers into an organic electrolytic solution, or an electrolyte layer having cross-linked polymers obtained by allowing electrolyte-containing liquid polymers to undergo polymerization reaction.
However, in the electrochromic display device formed of pixel electrodes arranged in a matrix and configured to display or dissipate desired pixels, electric charges are likely to diffuse outside of the selected pixel regions within the electrolyte layer. In particular, if the electrolyte layer is a liquid electrolyte layer, electric charges are more likely to diffuse outside of the selected pixel regions. To overcome such charge diffusion in the electrolyte layer, Japanese Patent Application Publication No. 2008-304906 (Patent Document 3) proposes a display device capable of selectively displaying desired pixels alone. In this display device, the electrolyte layer is formed corresponding to the selected pixels to be displayed to prevent the electric charges from diffusing outside of the selected pixel regions.
The electrochromic display devices may develop various colors based on various structures of electrochromic compounds, and hence, they are expected to be utilized as multicolor display device devices. An example of the multicolor display device utilizing the electrochromic display device is disclosed in Japanese Patent Application Publication No. 2003-270671 (Patent Document 4). The disclosed multicolor display device includes two or more layers of electrochromic elements each having a structural unit formed by arranging an electrochromic layer and an electrolyte layer between a pair of transparent electrodes.
In addition, Japanese Patent Application Publication No. 2010-33016 (Patent Document 5) discloses another example of the multicolor display device utilizing the electrochromic display. The disclosed multicolor display device is formed by two or more electrochromic layers between a pair of a display substrate and a counter electrode. In the disclosed multicolor display device, two or more display electrodes are provided mutually separate from each other between the pair of the display substrate and the counter electrode so that the electrochromic layer is formed corresponding to each of the display electrodes.
However, the multicolor display devices utilizing the electrochromic display devices disclosed in the related art seem to have room for improvement in the following aspects.
For example, in the multicolor electrochromic display device disclosed in Patent Document 4, since the multicolor electrochromic display device is formed of the layers of two or more electrochromic elements, there may be an increase in manufacturing cost compared to that of the monochrome electrochromic display device formed of one layer of the electrochromic element. Further, since the monochrome electrochromic display device formed of one layer of the electrochromic element needs to have two layers of transparent electrodes, the multicolor electrochromic display formed of two or more layers of the electrochromic elements may need to have twice as many as the number of layers of the electrochromic elements, which may lower the reflectance and the contrast.
Meanwhile, in the multicolor electrochromic display device disclosed in Patent Document 5, the electrochromic layers of the display electrodes are selectively driven to develop or dissipate a corresponding color of the electrochromic layer of the selected display electrode. Accordingly, the electric resistance between the display electrodes arranged separately from each other may need to be higher than the electric resistance within each of the display electrode surfaces. That is, if the electric resistance between the display electrodes is small, current may flow into unselected display electrodes. That is, it may not be possible to drive the selected display electrodes to independently develop or dissipate colors of the electrochromic layers of the selected display electrodes.
However, in the electrochromic display device disclosed in Patent Document 5, it may be difficult to sufficiently obtain insulating properties between the display electrodes in driving the selected electrochromic layers corresponding to the counter electrodes to independently develop or dissipate the corresponding colors of the selected electrochromic layers (active matrix). That is, the preceding developed color of the selected electrochromic layer may have an adverse effect on the succeeding developing color operation for developing the selected color of the electrochromic layer to be developed.